Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic materials is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Several types of supports can be used for the manufacturing of a diazo-sensitized lithographic printing plate. Common supports are metal supports like Al or Zn and paper bases. These supports, if not sufficient hydrophilic by themselves, are first coated with a hydrophilic layer to form the hydrophilic background of the printing plate and a top layer containing the diazo compound is then applied (see for example DE-P-1900469, DE-P-2030634 and U.S. Pat. No. 3,971,660).
It is known to use as hydrophilic layer in these systems a layer containing polyvinyl alcohol and hydrolyzed tetraethyl orthosilicate and preferably also silicium dioxide and/or titanium dioxide as described in e.g. EP-A 601240, GB-P-1419512, FR-P-2300354, U.S. Pat. Nos. 3,971,660 and 4,284,705. This hydrophilic layer is overcoated with a light-sensitive layer containing a diazo resin or a diazonium salt in a polymeric binder.
Upon image-wise exposure of the light-sensitive layer the exposed image areas become water insoluble and the unexposed areas remain water soluble. The plate is then developed with water to remove the diazonium salt or diazo resin in the unexposed areas.
Commercially available diazo based printing plates most commonly use an anodized and roughened aluminum as a support having a hydrophilic surface. However, commercial plates are also available that use a flexible support such as paper provided with a hydrophilic layer. For example, Lithocraft 10008 FOTOPLATE.TM. is a diazo based printing plate that comprises on a paper support a hydrophilic layer on top of which is provided a diazo based photosensitive layer. According to plate instructions of the supplier, a plate can be prepared by image-wise exposure of the lithographic printing plate precursor or imaging element, mounting the exposed imaging element on the press and wiping its surface with Lithocraft.RTM. 10008 Developer Desensitizer. The plate instructions also contemplate a method wherein no developer desensitizer is used. However, such method most often results in poor lithographic preformance so that in practice a Developer Desensitizer is almost always needed.
EP-A 601240 discloses a diazo based printing plate that uses a polyester film provided with a cross-linked hydrophilic layer as a lithographic base on which a photosensitive diazo layer is provided. Such a diazo based printing plate can be developed by rinsing it with plain water subsequent to image-wise exposure. However, in practical working conditions, where printing plates that are prepared are stored for some time and are often dried before mounting on the press, it was found that lithographic performance does not always meets the demands and in particular substantial dot gain and unwanted ink acceptance may occur. The dot gain is of particular concern as it may vary from one plate to the other making it difficult to compensate for it during plate preparation. Moreover, during development a substantial amount of liquid waste is generated.